Here's a "W" charm that I bought at a thrift store for $1. I took a chance on it, since the marking says 4K (maybe the 1 didn't get stuck on the charm). I acid tested the charm and based on the instruction. It tested at 12 K gold. The top line (drop) remained perfect, which was the 10 K acid. So the charm was at least 10 K gold. When I tested it with the 14 K acid, the gold should have been dissolved in the drop and disappeared. The bottom drop shows the 14 K acid on the scratch line. Noticed, that it didn't dissolve. Instead it turned a brown/reddish line, which the directions said means the gold value is one step less, in this case 12 K. So it is a 12 K gold Charm, I bought for a buck.
Good Luck and Happy Hunting,
Author of "Cheap Gold and Silver"